Remove Squeeze-only workaround for the HashType enum in Tails::IUK::TargetFile::Download

In the iuk repository, lib/Tails/IUK/TargetFile/Download.pm reads: “FIXME (Wheezy): Squeeze’s Moose::Util::TypeConstraints absolutely wants at least two members in an enum”. We should remove the ugly placeholder in that enum.

Related issues

Blocked by #6015 (closed)

Original created by @intrigeri on 7042 (Redmine)

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